There was a subtle but important shift in David Cameron's political message today. For the first time, he started to set himself up as an economic war leader. It's the beginning of a new storyline, one which is connected to, but different from, the austerity message we have seen so far.

Making a major speech on the economy, the prime minister said: "We are living in perilous economic times. Turn on the TV news and you see the return of a crisis that never really went away: Greece on the brink, the survival of the euro in question. Faced with this, I have a clear task: to keep Britain safe. Not to take the easy course - but the right course... That is why we must resist dangerous voices calling on us to retreat... It’s not an alternative policy, it's a cop-out."

Cameron's speech was the first moment in a gradual moving of the goalposts. Soon, the function of the government will not be to improve economic performance, but merely to protect Britain from the eurozone crisis.

This change in narrative has five important advantages to the government.

Firstly, it makes opposition policy not wrong, but irresponsible – even unpatriotic. Note the way the final two sentences in the quote above contrasts the effort to "keep Britain safe" to Ed Miliband's economic argument. The speech dresses up Labour and Conservative economic differences not as policy distinctions, but as a national security issue.

Secondly, it excuses the British government's dire economic performance. George Osborne had almost run out of excuses when the eurozone crisis hit the front pages. From there on in, he has relied on it to explain away the double-dip recession, despite the fact that France and Germany, who are both in the eurozone, have still managed to clock up minor growth.

Third, by focusing only on protecting Britain from the crisis, Cameron has lowered the benchmark for success. When Cameron and Osborne entered Downing Street, they promised economic growth, with the private sector thriving while the state was cut back. You may have noticed this did not transpire. It is unlikely to do so even by the 2015 general election. Promising merely to protect the country from the eurozone crisis means Cameron has merely to uphold the status quo. It is an entirely negative accomplishment – he means to prevent, rather than accomplish.

Of course, many of the greatest accomplishments of British prime ministers have been about prevention. Harold Wilson managed to keep Britain out of Vietnam, for instance. He was hated by US president Lyndon Johnson and despised by the radical British students who wanted him to condemn the conflict. Nobody credits him much for it even now. But it was one of the greatest achievements of any post-war British prime minister. Prevention can be a great accomplishment, but let's at least notice that it is a substantially lower benchmark than actually improving the economy.

Focusing on the eurozone crisis provides two other benefits to Cameron. It gives a singular narrative to a coalition government which appears increasingly aimless. Tory backbenchers seem to enjoy fighting each other much more than Labour. Liberal Democrats are trying to figure out how to survive the existential threat of government. Government policy has become so stretched with competing demands from the political centre to the hard right that no coherent argument could explain it.

Cameron recognises the danger. He has been desperate to define the government by something more than austerity, hence his obsession with the 'big society'. A war narrative – 'Britain protected against the chaos of Europe' – provides an instant narrative and an emotionally resonant one. It even harks back to the slogan the Tories used during their first post-election conference: 'Together in the national interest'. It unifies rebellious Tory backbenchers with centrist Lib Dem ministers and gives the public a clear idea of what the government is actually for.

Finally, the creation of a war narrative tends to favour the incumbent. In general, people will stick with what they know when there is chaos swirling around them. They will be less likely to take a chance on a Labour economic policy in which they are only now starting to show some interest.

If Cameron can make that war narrative stick, he'll stick to it like a drowning man to a lifeboat.

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